Chapter 2. Asynchronous I/O based on NIO

2.1. Differences from other I/O frameworks

Solves similar problems as other frameworks, but has certain distinct features:

minimalistic, optimized for data volume intensive protocols such as HTTP.

efficient memory management: data consumer can read is only as much input data as it
can process without having to allocate more memory.

direct access to the NIO channels where possible.

2.2. I/O reactor

HttpCore NIO is based on the Reactor pattern as described by Doug Lea. The purpose of I/O
reactors is to react to I/O events and to dispatch event notifications to individual I/O
sessions. The main idea of I/O reactor pattern is to break away from the one thread per
connection model imposed by the classic blocking I/O model. The IOReactor
interface represents an abstract object which implements the Reactor pattern.
Internally, IOReactor implementations encapsulate
functionality of the NIO java.nio.channels.Selector.

I/O reactors usually employ a small number of dispatch threads (often as few as one) to
dispatch I/O event notifications to a much greater number (often as many as several
thousands) of I/O sessions or connections. It is generally recommended to have one dispatch
thread per CPU core.

2.2.1. I/O dispatchers

IOReactor implementations make use of the
IOEventDispatch interface to notify clients of events
pending for a particular session. All methods of the IOEventDispatch
are executed on a dispatch thread of the I/O reactor. Therefore, it is
important that processing that takes place in the event methods will not block the
dispatch thread for too long, as the I/O reactor will be unable to react to other
events.

2.2.2. I/O reactor shutdown

The shutdown of I/O reactors is a complex process and may usually take a while to
complete. I/O reactors will attempt to gracefully terminate all active I/O sessions and
dispatch threads approximately within the specified grace period. If any of the I/O
sessions fails to terminate correctly, the I/O reactor will forcibly shut down
remaining sessions.

2.2.3. I/O sessions

The IOSession interface represents a sequence of
logically related data exchanges between two end points. IOSession
encapsulates functionality of NIO
java.nio.channels.SelectionKey and
java.nio.channels.SocketChannel. The channel associated with the
IOSession can be used to read data from and write data
to the session.

2.2.4. I/O session state management

I/O sessions are not bound to an execution thread, therefore one cannot use the context
of the thread to store a session's state. All details about a particular session must
be stored within the session itself.

Event notifications will not take place if the corresponding interest flag is not set.

2.2.6. I/O session buffers

Quite often I/O sessions need to maintain internal I/O buffers in order to transform
input / output data prior to returning it to the consumer or writing it to the
underlying channel. Memory management in HttpCore NIO is based on the fundamental
principle that the data a consumer can read, is only as much input data as it can process
without having to allocate more memory. That means, quite often some input data may
remain unread in one of the internal or external session buffers. The I/O reactor can
query the status of these session buffers, and make sure the consumer gets notified
correctly as more data gets stored in one of the session buffers, thus allowing the
consumer to read the remaining data once it is able to process it. I/O sessions can be
made aware of the status of external session buffers using the
SessionBufferStatus interface.

2.2.7. I/O session shutdown

One can close an I/O session gracefully by calling IOSession#close()
allowing the session to be closed in an orderly manner or by calling
IOSession#shutdown() to forcibly close the underlying channel.
The distinction between two methods is of primary importance for those types of I/O
sessions that involve some sort of a session termination handshake such as SSL/TLS
connections.

2.2.8. Listening I/O reactors

ListeningIOReactor represents an I/O reactor capable of
listening for incoming connections on one or several ports.

Opening a connection to a remote host usually tends to be a time consuming process and
may take a while to complete. One can monitor and control the process of session
initialization by means of the SessionRequestinterface.

// Make sure the request times out if connection
// has not been established after 1 sec
sessionRequest.setConnectTimeout(1000);
// Wait for the request to complete
sessionRequest.waitFor();
// Has request terminated due to an exception?
if (sessionRequest.getException() != null) {
sessionRequest.getException().printStackTrace();
}
// Get hold of the new I/O session
IOSession iosession = sessionRequest.getSession();

SessionRequest implementations are expected to be
thread-safe. Session request can be aborted at any time by calling
IOSession#cancel() from another thread of execution.

if (!sessionRequest.isCompleted()) {
sessionRequest.cancel();
}

One can pass several optional parameters to the
ConnectingIOReactor#connect() method to exert a greater control over the
process of session initialization.

A non-null local socket address parameter can be used to bind the socket to a specific
local address.

It is often desirable to be able to react to the completion of a session request
asynchronously without having to wait for it, blocking the current thread of execution.
One can optionally provide an implementation SessionRequestCallback
interface to get notified of events related to session requests, such
as request completion, cancellation, failure or timeout.

2.3.1. Queuing of I/O interest set operations

Several older JRE implementations (primarily from IBM) include what Java API
documentation refers to as a naive implementation of the
java.nio.channels.SelectionKey class. The problem with
java.nio.channels.SelectionKey in such JREs is that reading or writing
of the I/O interest set may block indefinitely if the I/O selector is in the process
of executing a select operation. HttpCore NIO can be configured to operate in a special
mode wherein I/O interest set operations are queued and executed by on the dispatch
thread only when the I/O selector is not engaged in a select operation.

2.4. I/O reactor exception handling

Protocol specific exceptions as well as those I/O exceptions thrown in the course of
interaction with the session's channel are to be expected and are to be dealt with by specific
protocol handlers. These exceptions may result in termination of an individual session but
should not affect the I/O reactor and all other active sessions. There are situations,
however, when the I/O reactor itself encounters an internal problem such as an I/O
exception in the underlying NIO classes or an unhandled runtime exception. Those types of
exceptions are usually fatal and will cause the I/O reactor to shut down automatically.

There is a possibility to override this behavior and prevent I/O reactors from shutting
down automatically in case of a runtime exception or an I/O exception in internal classes.
This can be accomplished by providing a custom implementation of the
IOReactorExceptionHandler interface.

One needs to be very careful about discarding exceptions indiscriminately. It is often much
better to let the I/O reactor shut down itself cleanly and restart it rather than leaving
it in an inconsistent or unstable state.

2.4.1. I/O reactor audit log

If an I/O reactor is unable to automatically recover from an I/O or a runtime exception
it will enter the shutdown mode. First off, it will close all active listeners and
cancel all pending new session requests. Then it will attempt to close all active I/O
sessions gracefully giving them some time to flush pending output data and terminate
cleanly. Lastly, it will forcibly shut down those I/O sessions that still remain active
after the grace period. This is a fairly complex process, where many things can fail at
the same time and many different exceptions can be thrown in the course of the shutdown
process. The I/O reactor will record all exceptions thrown during the shutdown process,
including the original one that actually caused the shutdown in the first place, in an
audit log. One can examine the audit log and decide whether it is safe to restart the
I/O reactor.

2.5. Non-blocking HTTP connections

Effectively non-blocking HTTP connections are wrappers around IOSession
with HTTP specific functionality. Non-blocking HTTP connections are
stateful and not thread-safe. Input / output operations on non-blocking HTTP connections
should be restricted to the dispatch events triggered by the I/O event dispatch thread.

2.5.1. Execution context of non-blocking HTTP connections

Non-blocking HTTP connections are not bound to a particular thread of execution and
therefore they need to maintain their own execution context. Each non-blocking HTTP
connection has an HttpContext instance associated with
it, which can be used to maintain a processing state. The HttpContext
instance is thread-safe and can be manipulated from multiple threads.

2.5.2. Working with non-blocking HTTP connections

At any point of time one can obtain the request and response objects currently being
transferred over the non-blocking HTTP connection. Any of these objects, or both, can
be null if there is no incoming or outgoing message currently being transferred.

However, please note that the current request and the current response may not
necessarily represent the same message exchange! Non-blocking HTTP connections can
operate in a full duplex mode. One can process incoming and outgoing messages
completely independently from one another. This makes non-blocking HTTP connections
fully pipelining capable, but at same time implies that this is the job of the protocol
handler to match logically related request and the response messages.

Over-simplified process of submitting a request on the client side may look like this:

Please note that one should rarely need to transmit messages using these low level
methods and should use appropriate higher level HTTP service implementations instead.

2.5.3. HTTP I/O control

All non-blocking HTTP connections classes implement IOControl
interface, which represents a subset of connection functionality for
controlling interest in I/O even notifications. IOControl
instances are expected to be fully thread-safe. Therefore
IOControl can be used to request / suspend I/O event
notifications from any thread.

One must take special precautions when interacting with non-blocking connections.
HttpRequest and HttpResponse
are not thread-safe. It is generally advisable that all input / output
operations on a non-blocking connection are executed from the I/O event dispatch
thread.

The following pattern is recommended:

Use IOControl interface to pass control over
connection's I/O events to another thread / session.

If input / output operations need be executed on that particular connection,
store all the required information (state) in the connection context and
request the appropriate I/O operation by calling
IOControl#requestInput() or IOControl#requestOutput()
method.

Execute the required operations from the event method on the dispatch thread
using information stored in connection context.

Please note all operations that take place in the event methods should not block for
too long, because while the dispatch thread remains blocked in one session, it is
unable to process events for all other sessions. I/O operations with the underlying
channel of the session are not a problem as they are guaranteed to be non-blocking.

2.5.4. Non-blocking content transfer

The process of content transfer for non-blocking connections works completely
differently compared to that of blocking connections, as non-blocking connections need
to accommodate to the asynchronous nature of the NIO model. The main distinction
between two types of connections is inability to use the usual, but inherently blocking
java.io.InputStream and java.io.OutputStream
classes to represent streams of inbound and outbound content. HttpCore NIO
provides ContentEncoder and
ContentDecoder interfaces to handle the process of asynchronous content
transfer. Non-blocking HTTP connections will instantiate the appropriate implementation
of a content codec based on properties of the entity enclosed with the message.

Please note, one still has to provide an HttpEntity instance when submitting an entity
enclosing message to the non-blocking HTTP connection. Properties of that entity will
be used to initialize an ContentEncoder instance to be
used for transferring entity content. Non-blocking HTTP connections, however, ignore
inherently blocking HttpEntity#getContent() and
HttpEntity#writeTo() methods of the enclosed entities.

Likewise, incoming entity enclosing message will have an HttpEntity
instance associated with them, but an attempt to call
HttpEntity#getContent() or HttpEntity#writeTo()
methods will cause an java.lang.IllegalStateException. The
HttpEntity instance can be used to determine properties
of the incoming entity such as content length.

2.5.5. Supported non-blocking content transfer mechanisms

Default implementations of the non-blocking HTTP connection interfaces support three
content transfer mechanisms defined by the HTTP/1.1 specification:

Content-Length delimited:
The end of the content entity is determined by the value of the
Content-Length header. Maximum entity length:
Long#MAX_VALUE.

Identity coding:
The end of the content entity is demarcated by closing the underlying
connection (end of stream condition). For obvious reasons the identity encoding
can only be used on the server side. Max entity length: unlimited.

Chunk coding:
The content is sent in small chunks. Max entity length: unlimited.

The appropriate content codec will be created automatically depending on properties of
the entity enclosed with the message.

2.5.6. Direct channel I/O

Content codes are optimized to read data directly from or write data directly to the
underlying I/O session's channel, whenever possible avoiding intermediate buffering in
a session buffer. Moreover, those codecs that do not perform any content transformation
(Content-Length delimited and identity codecs, for example) can
leverage NIO java.nio.FileChannel methods for significantly
improved performance of file transfer operations both inbound and outbound.

If the actual content decoder implements FileContentDecoder
one can make use of its methods to read incoming content directly to a
file bypassing an intermediate java.nio.ByteBuffer.

Server side HTTP I/O events as defined by the NHttpServerEventHandler
interface:

connected:
Triggered when a new incoming connection has been created.

requestReceived:
Triggered when a new HTTP request is received. The connection passed as a parameter
to this method is guaranteed to return a valid HTTP request object. If the request
received encloses a request entity this method will be followed a series of
inputReady events to transfer the request content.

inputReady:
Triggered when the underlying channel is ready for reading a new portion of
the request entity through the corresponding content decoder. If the content
consumer is unable to process the incoming content, input event notifications can
temporarily suspended using IOControl interface
(super interface of NHttpServerConnection).
Please note that the NHttpServerConnection and
ContentDecoder objects are not thread-safe and
should only be used within the context of this method call. The
IOControl object can be shared and used on other thread to resume
input event notifications when the handler is capable of processing more content.

responseReady:
Triggered when the connection is ready to accept new HTTP response. The protocol
handler does not have to submit a response if it is not ready.

outputReady:
Triggered when the underlying channel is ready for writing a next portion of the
response entity through the corresponding content encoder. If the content producer
is unable to generate the outgoing content, output event notifications can be
temporarily suspended using IOControl interface
(super interface of NHttpServerConnection).
Please note that the NHttpServerConnection and
ContentEncoder objects are not thread-safe and
should only be used within the context of this method call. The
IOControl object can be shared and used on other thread to resume
output event notifications when more content is made available.

exception:
Triggered when an I/O error occurrs while reading from or writing to the underlying
channel or when an HTTP protocol violation occurs while receiving an HTTP request.

timeout:
Triggered when no input is detected on this connection over the maximum period of
inactivity.

closed:
Triggered when the connection has been closed.

Client side HTTP I/O events as defined by the NHttpClientEventHandler
interface:

connected:
Triggered when a new outgoing connection has been created. The attachment object
passed as a parameter to this event is an arbitrary object that was attached to
the session request.

requestReady:
Triggered when the connection is ready to accept new HTTP request. The protocol
handler does not have to submit a request if it is not ready.

outputReady:
Triggered when the underlying channel is ready for writing a next portion of the
request entity through the corresponding content encoder. If the content producer
is unable to generate the outgoing content, output event notifications can be
temporarily suspended using IOControl interface
(super interface of NHttpClientConnection).
Please note that the NHttpClientConnection and
ContentEncoder objects are not thread-safe and
should only be used within the context of this method call. The
IOControl object can be shared and used on other thread to resume
output event notifications when more content is made available.

responseReceived:
Triggered when an HTTP response is received. The connection passed as a parameter to
this method is guaranteed to return a valid HTTP response object. If the response
received encloses a response entity this method will be followed a series of
inputReady events to transfer the response content.

inputReady:
Triggered when the underlying channel is ready for reading a new portion of the
response entity through the corresponding content decoder. If the content consumer
is unable to process the incoming content, input event notifications can be
temporarily suspended using IOControl interface
(super interface of NHttpClientConnection).
Please note that the NHttpClientConnection and
ContentDecoder objects are not thread-safe and
should only be used within the context of this method call. The
IOControl object can be shared and used on other thread to resume
input event notifications when the handler is capable of processing more content.

exception:
Triggered when an I/O error occurs while reading from or writing to the underlying
channel or when an HTTP protocol violation occurs while receiving an HTTP response.

timeout:
Triggered when no input is detected on this connection over the maximum period of
inactivity.

closed:
Triggered when the connection has been closed.

2.7. Non-blocking HTTP content producers

As discussed previously the process of content transfer for non-blocking connections works
completely differently compared to that for blocking connections. For obvious reasons
classic I/O abstraction based on inherently blocking java.io.InputStream
and java.io.OutputStream classes is not well suited
for asynchronous data transfer. In order to avoid inefficient and potentially blocking
I/O operation redirection through java.nio.channels.Channles#newChannel
non-blocking HTTP entities are expected to implement NIO specific extension
interface HttpAsyncContentProducer.

The HttpAsyncContentProducer interface defines several
additional method for efficient streaming of content to a non-blocking HTTP connection:

produceContent:
Invoked to write out a chunk of content to the ContentEncoder
. The IOControl interface can be
used to suspend output events if the entity is temporarily unable to produce more
content. When all content is finished, the producer MUST call
ContentEncoder#complete(). Failure to do so may cause
the entity to be incorrectly delimited.
Please note that the ContentEncoder object is
not thread-safe and should only be used within the context of this method call.
The IOControl object can be shared and used on other
thread resume output event notifications when more content is made available.

isRepeatable:
Determines whether or not this producer is capable of producing its content more
than once. Repeatable content producers are expected to be able to recreate
their content even after having been closed.

close:
Closes the producer and releases all resources currently allocated by it.

2.7.1. Creating non-blocking entities

Several HTTP entity implementations included in HttpCore NIO support
HttpAsyncContentProducer interface:

2.7.1.1. NByteArrayEntity

This is a simple self-contained repeatable entity, which receives its content from
a given byte array. This byte array is supplied to the constructor.

NByteArrayEntity entity = new NByteArrayEntity(new byte[] {1, 2, 3});

2.7.1.2. NStringEntity

This is a simple, self-contained, repeatable entity that retrieves its data from a
java.lang.String object. It has 2 constructors, one simply
constructs with a given string where the other also takes a character encoding for
the data in the java.lang.String.

2.7.1.3. NFileEntity

This entity reads its content body from a file. This class is mostly used to stream
large files of different types, so one needs to supply the content type of the file
to make sure the content can be correctly recognized and processed by the
recipient.

Upon receiving an incoming request the HttpAsyncService
verifies the message for compliance with the server expectations using
HttpAsyncExpectationVerifier, if provided, and then
HttpAsyncRequestHandlerResolver is used to resolve the request URI to
a particular HttpAsyncRequestHandler intended to handle
the request with the given URI. The protocol handler uses the selected
HttpAsyncRequestHandler instance to process the incoming request and
to generate an outgoing response.

2.8.1.1. Non-blocking HTTP request handlers

HttpAsyncRequestHandler represents a routine for
asynchronous processing of a specific group of non-blocking HTTP requests.
Protocol handlers are designed to take care of protocol specific aspects, whereas
individual request handlers are expected to take care of application specific HTTP
processing. The main purpose of a request handler is to generate a response object
with a content entity to be sent back to the client in response to the given
request.

Request handlers must be implemented in a thread-safe manner. Similarly to
servlets, request handlers should not use instance variables unless access to those
variables are synchronized.

2.8.1.2. Asynchronous HTTP exchange

The most fundamental difference of the non-blocking request handlers compared to
their blocking counterparts is ability to defer transmission of the HTTP response
back to the client without blocking the I/O thread by delegating the process of
handling the HTTP request to a worker thread or another service. The instance of
HttpAsyncExchange passed as a parameter to the
HttpAsyncRequestHandler#handle method to submit
a response as at a later point once response content becomes available.

The HttpAsyncExchange interface can be interacted
with using the following methods:

getRequest:
Returns the received HTTP request message.

getResponse:
Returns the default HTTP response message that can submitted once ready.

submitResponse:
Submits an HTTP response and completed the message exchange.

isCompleted:
Determines whether or not the message exchange has been completed.

setCallback:
Sets Cancellable callback to be invoked
in case the underlying connection times out or gets terminated prematurely
by the client. This callback can be used to cancel a long running response
generating process if a response is no longer needed.

Please note HttpResponse instances are not
thread-safe and may not be modified concurrently. Non-blocking request handlers
must ensure HTTP response cannot be accessed by more than one thread at a time.

2.8.1.3. Asynchronous HTTP request consumer

HttpAsyncRequestConsumer facilitates the process of
asynchronous processing of HTTP requests. It is a callback interface used by
HttpAsyncRequestHandlers to process an incoming
HTTP request message and to stream its content from a non-blocking server side
HTTP connection.

HTTP I/O events and methods as defined by the
HttpAsyncRequestConsumer interface:

requestReceived:
Invoked when a HTTP request message is received.

consumeContent:
Invoked to process a chunk of content from the
ContentDecoder. The IOControl
interface can be used to suspend input events if
the consumer is temporarily unable to consume more content.
The consumer can use the ContentDecoder#isCompleted()
method to find out whether or not the message content
has been fully consumed.
Please note that the ContentDecoder object
is not thread-safe and should only be used within the context of this
method call. The IOControl object can be
shared and used on other thread to resume input event notifications
when the consumer is capable of processing more content.
This event is invoked only if the incoming request message has
a content entity enclosed in it.

requestCompleted:
Invoked to signal that the request has been fully processed.

failed:
Invoked to signal that the request processing terminated abnormally.

getException:
Returns an exception in case of an abnormal termination. This method
returns null if the request execution is still ongoing or if
it completed successfully.

getResult:
Returns a result of the request execution, when available. This method
returns null if the request execution is still ongoing.

isDone:
Determines whether or not the request execution completed. If the
request processing terminated normally getResult()
can be used to obtain the result. If the request processing terminated
abnormally getException() can be used to obtain
the cause.

close:
Closes the consumer and releases all resources currently allocated by it.

HttpAsyncRequestConsumer implementations are
expected to be thread-safe.

BasicAsyncRequestConsumer is a very basic implementation
of the HttpAsyncRequestConsumer interface shipped
with the library. Please note that this consumer buffers request content in memory and
therefore should be used for relatively small request messages.

2.8.1.4. Asynchronous HTTP response producer

HttpAsyncResponseProducer facilitates the process of
asynchronous generation of HTTP responses. It is a callback interface used by
HttpAsyncRequestHandlers to generate an HTTP response
message and to stream its content to a non-blocking server side HTTP connection.

HTTP I/O events and methods as defined by the
HttpAsyncResponseProducer interface:

generateResponse:
Invoked to generate a HTTP response message header.

produceContent:
Invoked to write out a chunk of content to the
ContentEncoder. The IOControl
interface can be used to suspend output events if
the producer is temporarily unable to produce more content.
When all content is finished, the producer MUST call
ContentEncoder#complete(). Failure to do so may cause
the entity to be incorrectly delimited.
Please note that the ContentEncoder object
is not thread-safe and should only be used within the context of this
method call. The IOControl object can be
shared and used on other thread resume output event notifications when
more content is made available.
This event is invoked only for if the outgoing response message has
a content entity enclosed in it, that is
HttpResponse#getEntity() returns null.

responseCompleted:
Invoked to signal that the response has been fully written out.

failed:
Invoked to signal that the response processing terminated abnormally.

close:
Closes the producer and releases all resources currently allocated by it.

HttpAsyncResponseProducer implementations are
expected to be thread-safe.

BasicAsyncResponseProducer is a basic implementation
of the HttpAsyncResponseProducer interface shipped
with the library. The producer can make use of the
HttpAsyncContentProducer interface to efficiently stream out
message content to a non-blocking HTTP connection, if it is implemented by the
HttpEntity enclosed in the response.

2.8.1.5. Non-blocking request handler resolver

The management of non-blocking HTTP request handlers is quite similar to that of
blocking HTTP request handlers. Usually an instance of
HttpAsyncRequestHandlerResolver is used to maintain a registry of
request handlers and to matches a request URI to a particular request handler.
HttpCore includes only a very simple implementation of the request handler resolver
based on a trivial pattern matching algorithm:
HttpAsyncRequestHandlerRegistry supports only three formats:
*, <uri>* and
*<uri>.

HttpAsyncRequestExecutor relies on
HttpAsyncRequestExecutionHandler to implement application specific
content generation and processing and to handle logically related series of HTTP
request / response exchanges, which may also span across multiple connections.
HttpProcessor provided by the
HttpAsyncRequestExecutionHandler instance will be used to generate
mandatory protocol headers for all outgoing messages and apply common, cross-cutting
message transformations to all incoming and outgoing messages. The caller is expected
to pass an instance of HttpAsyncRequestExecutionHandler
to be used for the next series of HTTP message exchanges through the connection
context using HttpAsyncRequestExecutor#HTTP_HANDLER attribute.
HTTP exchange sequence is considered complete when the
HttpAsyncRequestExecutionHandler#isDone() method returns true.

The HttpAsyncRequester utility class can be used to abstract
away low level details of HttpAsyncRequestExecutionHandler
management. Please note HttpAsyncRequester
supports single HTTP request / response exchanges only. It does not support HTTP
authentication and does not handle redirects automatically.

2.8.2.1. Asynchronous HTTP request producer

HttpAsyncRequestProducer facilitates the process of
asynchronous generation of HTTP requests. It is a callback interface whose methods
get invoked to generate an HTTP request message and to stream message content to
a non-blocking client side HTTP connection.

Repeatable request producers capable of generating the same request message more
than once can be reset to their initial state by calling the
resetRequest() method, at which point request producers are expected
to release currently allocated resources that are no longer needed or re-acquire
resources needed to repeat the process.

HTTP I/O events and methods as defined by the
HttpAsyncRequestProducer interface:

getTarget:
Invoked to obtain the request target host.

generateRequest:
Invoked to generate a HTTP request message header. The message is expected
to implement the HttpEntityEnclosingRequest
interface if it is to enclose a content entity.

produceContent:
Invoked to write out a chunk of content to the
ContentEncoder. The IOControl
interface can be used to suspend output events if
the producer is temporarily unable to produce more content.
When all content is finished, the producer MUST call
ContentEncoder#complete(). Failure to do so may cause
the entity to be incorrectly delimited
Please note that the ContentEncoder object
is not thread-safe and should only be used within the context of this
method call. The IOControl object can be
shared and used on other thread resume output event notifications when
more content is made available.
This event is invoked only for if the outgoing request message has
a content entity enclosed in it, that is
HttpEntityEnclosingRequest#getEntity() returns null
.

requestCompleted:
Invoked to signal that the request has been fully written out.

failed:
Invoked to signal that the request processing terminated abnormally.

resetRequest:
Invoked to reset the producer to its initial state. Repeatable request
producers are expected to release currently allocated resources that are
no longer needed or re-acquire resources needed to repeat the process.

close:
Closes the producer and releases all resources currently allocated by it.

HttpAsyncRequestProducer implementations are
expected to be thread-safe.

BasicAsyncRequestProducer is a basic implementation
of the HttpAsyncRequestProducer interface shipped
with the library. The producer can make use of the
HttpAsyncContentProducer interface to efficiently stream out
message content to a non-blocking HTTP connection, if it is implemented by the
HttpEntity enclosed in the request.

2.8.2.2. Asynchronous HTTP response consumer

HttpAsyncResponseConsumer facilitates the process of
asynchronous processing of HTTP responses. It is a callback interface whose methods
get invoked to process an HTTP response message and to stream message content from
a non-blocking client side HTTP connection.

HTTP I/O events and methods as defined by the
HttpAsyncResponseConsumer interface:

responseReceived:
Invoked when a HTTP response message is received.

consumeContent:
Invoked to process a chunk of content from the
ContentDecoder. The IOControl
interface can be used to suspend input events if
the consumer is temporarily unable to consume more content.
The consumer can use the ContentDecoder#isCompleted()
method to find out whether or not the message content
has been fully consumed.
Please note that the ContentDecoder object
is not thread-safe and should only be used within the context of this
method call. The IOControl object can be
shared and used on other thread to resume input event notifications
when the consumer is capable of processing more content.
This event is invoked only for if the incoming response message has
a content entity enclosed in it.

responseCompleted:
Invoked to signal that the response has been fully processed.

failed:
Invoked to signal that the response processing terminated abnormally.

getException:
Returns an exception in case of an abnormal termination. This method
returns null if the response processing is still ongoing or
if it completed successfully.

getResult:
Returns a result of the response processing, when available. This method
returns null if the response processing is still ongoing.

isDone:
Determines whether or not the response processing completed. If the
response processing terminated normally getResult()
can be used to obtain the result. If the response processing terminated
abnormally getException() can be used to obtain
the cause.

close:
Closes the consumer and releases all resources currently allocated by it.

HttpAsyncResponseConsumer implementations are
expected to be thread-safe.

BasicAsyncResponseConsumer is a very basic implementation
of the HttpAsyncResponseConsumer interface shipped
with the library. Please note that this consumer buffers response content in memory
and therefore should be used for relatively small response messages.

2.9. Non-blocking connection pools

Non-blocking connection pools are quite similar to blocking one with one significant
distinction that they have to reply an I/O reactor to establish new connections.
As a result connections leased from a non-blocking pool are returned fully initialized and
already bound to a particular I/O session. Non-blocking connections managed by a connection
pool cannot be bound to an arbitrary I/O session.

Please note due to event-driven nature of asynchronous communication model it is quite
difficult to ensure proper release of persistent connections back to the pool. One can make
use of HttpAsyncRequester to handle connection lease and release
behind the scene.

2.10. Non-blocking TLS/SSL

2.10.1. SSL I/O session

SSLIOSession is a decorator class intended to transparently
extend any arbitrary IOSession with transport layer
security capabilities based on the SSL/TLS protocol. Default HTTP connection
implementations and protocol handlers should be able to work with SSL sessions without
special preconditions or modifications.

2.10.1.1. SSL setup handler

Applications can customize various aspects of the TLS/SSl protocol by passing a
custom implementation of the SSLSetupHandler
interface.

SSL events as defined by the SSLSetupHandler
interface:

initalize:
Triggered when the SSL connection is being initialized. The handler can use
this callback to customize properties of the
javax.net.ssl.SSLEngine used to establish the SSL session.

verify:
Triggered when the SSL connection has been established and initial SSL
handshake has been successfully completed. The handler can use this
callback to verify properties of the SSLSession. For instance this would
be the right place to enforce SSL cipher strength, validate certificate
chain and do hostname checks.

2.10.2. TLS/SSL aware I/O event dispatches

Default IOEventDispatch implementations shipped with
the library such as DefaultHttpServerIODispatch and
DefaultHttpClientIODispatch automatically detect SSL encrypted sessions
and handle SSL transport aspects transparently. However, custom I/O event dispatchers
that do not extend AbstractIODispatch are required to take some
additional actions to ensure correct functioning of the transport layer encryption.

The I/O dispatch may need to call SSLIOSession#initalize()
method in order to put the SSL session either into a client or
a server mode, if the SSL session has not been yet initialized.

When the underlying I/O session is input ready, the I/O dispatcher should check
whether the SSL I/O session is ready to produce input data by calling
SSLIOSession#isAppInputReady(), pass control to the
protocol handler if it is, and finally call
SSLIOSession#inboundTransport() method in order to do the
necessary SSL handshaking and decrypt input data.

When the underlying I/O session is output ready, the I/O dispatcher should
check whether the SSL I/O session is ready to accept output data by calling
SSLIOSession#isAppOutputReady(), pass control to the
protocol handler if it is, and finally call
SSLIOSession#outboundTransport() method in order to do the necessary
SSL handshaking and encrypt application data.